Friday, February 1, 2008
Pinophyta (or Coniferophyta) - Conifers Ginkgophyta - Ginkgo Cycadophyta - Cycads Gnetophyta - Gnetum, Ephedra, Welwitschia
The gymnosperms (Gymnospermae) are a group of spermatophyte seed-bearing plants with ovules on the edge or blade of an open sporophyll, the sporophylls usually arranged in cone-like structures. The other major group of seed-bearing plants, the angiosperms, have ovules enclosed in a carpel, a sporophyll with fused margins. The term gymnosperm comes from the Greek word gumnospermos, meaning "naked seeds" and referring to the unenclosed condition of the seeds, as when they are produced they are found naked on the scales of a cone or similar structure.
Gymnosperms are heterosporous, producing microspores that develop into pollen grains and megaspores that are retained in an ovule. After fertilization (joining of the micro- and megaspore), the resulting embryo, along with other cells comprising the ovule, develops into a seed. The seed is a sporophyte resting stage.
In early classification schemes, the gymnosperms (Gymnospermae) "naked seed" plants were regarded as a "natural" group. However, certain fossil discoveries suggest that the angiosperms evolved from a gymnosperm ancestor, which would make the gymnosperms a paraphyletic group if all extinct taxa are included. Modern cladistics only accepts taxa that are monophyletic, traceable to a common ancestor and inclusive of all descendants of that common ancestor. So, while the term 'gymnosperm' is still widely used for non-angiosperm seed-bearing plants, the plant species once treated as gymnosperms are usually distributed among four groups, which can be given equal rank as divisions within the Kingdom Plantae.
With regard to extant gymnosperms, molecular phylogenies of living taxa have conflicted with morphological datasets with regard to whether they comprise a monophyletic or paraphyletic group with respect to angiosperms. At issue is whether the Gnetophyta are the sister taxon to angiosperms, or whether they are sister to, or nested within, other extant gymnosperms.
Posted by gigihong07 at 7:29 AM